1. Field of the Invention
The present invention relates to a biomedical signal instrumentation amplifier and particularly to a voltage instrumentation amplifier provided with a biomedical signal level conversion circuit.
2. Description of Related Art
As technology changes with each passing day, a medical instrument is developed without exception. Because the technology of a very large scale integration (VLSI) and the concept of a System-on-a-Chip (SOC) are increasingly known, the valuable, large-scaled medical instrument is gradually being reduced and made more flexible so that it may be not only portable but also cheap and practical, thereby being widely used.
However, since late 1950's, the design and research of a biomedical signal detection system has been gradually regarded, and a biomedical signal instrument amplifier is even essential in the detection system. The biomedical signal is quite weak and different from a general electric signal, so the required biomedical signal instrumentation amplifier is different from a general amplifier.
The bandwidth of biomedical signal is part of low frequency; for example, the frequency of an electrocardiogram (ECG) signal ranges from 0.05 Hz to 100 Hz, that of an electroencephalogram (EEG) signal ranges from 0.5 Hz to 100 Hz, and that of an electro-oculogram (EOG) signal ranges from dc to 100 Hz.
The amplitude of biomedical signal is also quite weak, and where human beings stay is full of various noises that exist in the required bandwidth of biomedical signal. The noises must be filtered by a filter to reduce the impact on the biomedical signal.
When the biomedical signal from a human body is measured, a quite large common-mode voltage follows and thus in order to measure the biomedical signal that is not distorted, a biomedical signal instrumentation amplifier of an extremely high common mode rejection ratio (CMRR) is usually required to obtain the quite weak biomedical signal from a very high AC source noise.
The biomedical signal is very easily subject to the interference of an external signal and that of a physiology signal from a person under test, so it is more difficult to measure the signal, in which internal interference comprises noises of MOS and problems of wire layout in a circuit, and external interference comprises electromagnetic waves caused by AC source and noises caused by instable power. The physiology signal as interference comprises the EOG signal, myoelectric signal, the ECG signal and the like, and thus it is important to design the instrumentation amplifier of high CMRR to restrain the common mode noises and precisely amplify the minute biomedical signal.
Generally, the instrumentation amplifier may be on the whole divided into:    (1) a voltage-based instrumentation amplifier, and    (2) a current-based instrumentation amplifier.
Most technical literatures describe the design of voltage-based instrumentation amplifier. As shown in FIG. 1, the hardware is structured with 3 voltage-based operation amplifiers and 7 resistors as passive elements. A drawback is that the high CMRR is limited to the match capability of the resistors and the resistors must occupy a large area for precise values and their match capability at the time of wire layout.
Contrarily, a standard current-based instrumentation amplifier is formed with a current conveyor, of which the matching capability is not comparatively a problem but is accomplished requiring only a small number of resistors; the required passive elements are fewer for easy control of gain. Although the common mode rejection is higher than that of a conventional voltage-based instrumentation amplifier, the number of resistors and the size of each resistor relatively impacts on thermal noise; however, the entire circuit is huge. Thus, in order to effectively solve new problems derived from power consumption, circuit performance, and noises, a new structure must be provided to find an optimal circuit.
In the design of an instrument system, an amplifier is required to block a large interference noise signal that is a signal (common-mode signal) shared by two lead wires, and amplify a weak differential signal, in which the amplifier is generally called instrumentation amplifier.
As shown in FIG. 2, a conventional instrumentation amplifier is formed with a plurality of passive elements.
For achievement of the high-performance CMRR and power supply rejection ratio (PSRR), in addition to the requirements of low output impedance and high voltage gain and the requirement of perfect impedance matching of passive elements in the circuit, the conventional voltage-based instrumentation amplifier must satisfy the area requirement for the accuracy, so this invention is to provide the new structure to reduce the passive elements and the noises and meanwhile enhance the performance of circuit.
Consequently, because of the technical defects of described above, the applicant keeps on carving unflaggingly through wholehearted experience and research to develop the present invention, which can effectively improve the defects described above.